Radio transmission is used widely in various applications. In some applications, a signal is transmitted from a source to a destination for communication in that direction only. One common example is broadcast radio and broadcast television. When broadcast transmission is employed, a broadcaster can transmit a signal continuously for reception without receiving a return signal.
For bi-directional communication, each end of a communication path transmits and receives signals. Two common approaches to supporting bi-directional communication include time-division duplexing (TDD) and frequency-division duplexing (FDD). When a radio operates in TDD mode, it transmits (Tx) and receives (Rx) signals on the same frequency at different times. This is accomplished, for example, when the transmitted and received signals are within known timeslots by using a simple switch to switch between Tx mode of operation and Rx mode of operation for the appropriate timeslots. Advantageously, switches are easily implemented with limited loss and limited overall effect on switched signals.
An FDD radio, in contrast, transmits and receives at the same time but on different frequencies. These frequencies are often quite closely spaced. For example, cellular radio duplex spacing is only 60 MHz for a nominal 1.9 GHz radio. To avoid having the transmitter interfere with the receiver, FDD systems employ very sharp filters, so that any transmit energy that falls in the receive band is attenuated such that it does not interfere with the intended reception. These duplex filters have loss even in the pass-band but are necessary parts of the FDD system. The impact of the losses in transmission is, effectively, wasted RF energy that otherwise would have been provided to the antenna. The impact of the loss is expressed as a reduction in system power efficiency and, in the context of battery operated devices, a reduction in the usage time between battery charging cycles. On the reception side, filter losses mean that less signal energy is available for processing by the receiver and as a consequence the range of operation of the device in respect of distance is more limited.
While a same power amplifier (PA) could be used for either a TDD or FDD system, implementing a single system supporting both FDD and TDD and using a same PA is problematic. The extra duplexer losses incurred in implementing an FDD system—losses from the sharp filter, for example—unduly penalize a TDD system. Conversely, the switch used to toggle between Tx and Rx modes in a TDD system does not allow for FDD operation, since it will not provide the necessary duplex filtering.
In U.S. Pat. No. 5,881,369 in the name of Dean and Park and in U.S. Pat. No. 7,376,093 in the name of Barabash and Morris, dual mode, FDD-TDD, transceivers are disclosed. In order to achieve this, a plurality of additional switches are inserted within the circuit to support either FDD or TDD operation. For example, as shown in U.S. Pat. No. 7,376,093, an antenna is coupled to a first receive filter and first transmit filter in parallel one to another. Each filter is coupled to a first switch for coupling same to a receive and transmit path respectively for supporting FDD operation. When the first switches are closed, the receive signal and transmit signal are directed according to a conventional FDD transceiver. When the first switches are not both closed, closing them in an alternating sequence allows for a TDD transceiver. Of course, providing an additional switch to couple the two receive paths to allow all the received energy to reach the receiver is also shown in Barabash and Morris.
Unfortunately, as noted above, each of these configurations suffers losses associated with the filters in the receive path and the transmit path. Though overall, the configurations function, they provide reduced power efficiency and, in this regard, are often not cost effective dual mode solutions because of the operational cost imposed—reduced battery life, increased battery cost, etc.
It would be advantageous to overcome these and other limitations of the prior art.